1. Field of the Invention
This invention relates to a system that predicts the temperature of the voice coil in a loudspeaker using a thermal-model, and then using that information to perform appropriate compensation of the audio signal to reduce power compression and provide a desired frequency response across a desired bandwidth.
2. Related Art
An electromagnetic loudspeaker (transducer, motor, or driver) uses magnets to produce magnetic flux in an air gap. These magnets are typically permanent magnets, used in a magnetic circuit of ferromagnetic material to direct most of the flux produced by the permanent magnet through the components of the motor and into the air gap. A voice coil is placed in the air gap with its conductors wound cylindrically in a perpendicular orientation relative to the magnet generating the magnetic flux in the air gap. An audio amplifier is electrically connected to the voice coil to provide electrical signal that corresponds to a particular sound to the voice coil. The interaction between the electrical signal passing through the voice coil and the magnetic field produced by the permanent magnet causes the voice coil to oscillate in accordance with the electrical signal and, in turn, drives the diaphragm and produces sound.
One common problem associated with electromagnetic transducers is the generation and dissipation of heat. As current or electrical signal passes through the voice coil, the resistance of the conductive material of the voice coil generates heat in the voice coil. The tolerance of the transducer to heat is generally determined by the melting points of its various components and the heat capacity of the adhesive used to construct the voice coil. As the DC resistance of the voice coil comprises a major portion of a driver's impedance, most of the input power is converted into heat rather than sound. Thus, the power handling capacity of a driver is limited by its ability to tolerate heat. If more power is delivered to the transducer than it can handle, the transducer can burn up.
Another problem associated with heat generation is temperature-induced increase in resistance, commonly referred to as power compression. As the temperature of the voice coil increases, the DC resistance of copper or aluminum conductors or wires used in the transducer also increases. Put differently, as the voice coil gets hotter, the resistance of the voice coils changes. In other words, the resistance of the voice coil is not constant, rather the resistance of the voice coil goes up as the temperature goes up. This means that the voice coil draws less current or power as temperature goes up. Consequently, the power delivered to the loudspeaker may be less than what it should be depending on the temperature. For example, a voice coil made of copper may have a resistance of 6 ohms at room temperature; but a resistance of 12 ohms at 270° C. (520° F.). Therefore, at higher temperatures, the power output is reduced due to increased coil resistance.
In a typical single coil design using a ceramic magnet, the driver is very large and a heat sink is usually not employed. Accordingly, the temperature in the driver limits the power of the loudspeaker because the driver must not overheat. A common approach in the design of high power professional loudspeakers consists of simply making the driver structure large enough to dissipate the heat generated. Producing a high power speaker in this way results in very large and heavy speaker with a large driver structure to handle the heat generated. Thus, heat production is a major limiting factor in designing loudspeakers. There is a need to overcome the detrimental effect of heat and power compression in designing loudspeakers.